The present invention relates to induction devices and particularly to relatively large devices used for power generating and utilization having one or more distributed air gaps formed in the core. The distributed air gap is generally in the form of a magnetic particulate material in a matrix of dielectric material which can comprise a gas or a liquid or a solid or a semi-solid material or combinations thereof.
Induction devices such as reactors are used in power systems, for example, in order to compensate for the Ferranti effect from long overhead lines or extended cable systems causing high voltages in the open circuit or lightly loaded lines. Reactors are sometimes required to provide stability to long line systems. They may also be used for voltage control and switched into and out of the system during light load conditions. In a like manner, transformers are used in power systems to step up and step down voltages to useful levels.
Such devices are manufactured from similar components. Typically, one or more coils are wrapped around a laminated core to form windings, which may be coupled to the line or load and switched in and out of the circuit in a desirable manner. The equivalent magnetic circuit of a static inductive device comprises a source of magnetomotive force, which is a function of the number turns of the winding, in series with the reluctance of the core, which may include iron and, if provided, an air gap. While the air gap is not strictly speaking necessary, reactors and transformers without air gaps tend to saturate at high magnetic field densities. Thus, control is less precise and fault currents may produce catastrophic failures.
The core, shown in fragmentary form in FIG. 13, may be visualized as a body having a closed magnetic circuit, for example, a pair of legs and interconnecting yokes. One of the legs may be cut through to form the air gap. The core may support the windings which, when energized by a current, produces a magnetic field φ in the core, which extends across the air gap. At high current densities the magnetic field is intense.
Although useful and desirable, the gap represents a weak link in the structure of the core. The core tends to vibrate at a frequency twice that of the alternating input current. This is the source of vibrational noise and stress in such devices.
Another problem associated with the air gap is that the field φ fringes, spreads out and is less confined. Thus, field lines tend to enter and leave the core with a non-zero component transverse to the core laminations which can cause a concentration in unwanted eddy currents and hot spots in the core.
These problems are somewhat alleviated by the use of one or more inserts in the gap designed to stabilize the structure and thereby reduce vibrations. In addition, the structure, or insert, is formed of materials which are designed to reduce the fringing effects in the gap. However, these devices are difficult to manufacture and are expensive.
An article by Arthur W. Kelley and F. Peter Symonds of North Carolina State University entitled “Plastic-Iron-Powder-Distributed-Air-Gap Magnetic Material” discusses both discrete and distributed air gap inductor core technology as well as using fine metal powder in the making of specific shaped parts, such as air gap magnetic materials and also for use in making radar absorbing materials.
In the Kelley paper, the magnetic permeability is fixed and specific throughout the various applications disclosed. The present invention is directed to an air gap insert having a transitional zone wherein the magnetic permeability is at some intermediate value less than that of the core itself and greater than that of the air gap material itself.
The solutions presented in the Kelley article would only apply in the field of high frequency, low current signal handling and would not necessarily work in the field of high power, low frequency electronics.
The use of high power, low frequency inductors with air gaps have various problems associated with huge mechanical forces across the air gap as well as noise and vibration of the electrical devices. Such devices are also prone to energy losses and overheating in adjacent cores due to flux fringing. These problems are associated with high power, low frequency devices in part due to their large physical structure, something that is not present in the power electronic devices discussed in Kelley. Therefore, the solutions to these problems require very different solutions than those used to address the smaller devices of the power electronics field.
A typical insert comprises a cylindrical segment of radially laminated core steel plates arranged in a wedge shaped pattern. The laminated segments are molded in an epoxy resin as a solid piece or module. Ceramic spacers are placed on the surface of the module to space it from the core, or when multiple modules are used, from an adjacent module. In the latter case, the modules, and ceramic spacers are accurately stacked and cemented together to make a solid core limb for the device.
The magnetic field in the core creates pulsating forces across all air gaps which, in the case of devices used in power systems, can amount to hundreds of kilo-newtons (kN). The core must be stiff to eliminate these objectionable vibrations. The radial laminations in the modules reduce fringing flux entering flat surfaces of core steel which thereby reduce current overheating and hot spots.
These structures are difficult to build and require precise alignment of a number of specially designed laminated wedge shaped pieces to form the circular module. The machining must be precise and the ceramic spacers are likewise difficult to size and position accurately. As a result, such devices are relatively expensive. Accordingly, it is desirable to produce an air gap spacer which is of unitary construction and substantially less expensive than the described prior arrangements.